Dry static calorimeter for rf power measurement



Aug. 8, 1961 P. A. HUDSON ET AL DRY STATIC CALORIMETER FOR RF POWERMEASUREMENT 2 Sheets-Sheet 1 Filed NOV. 9, 1959 INVENTORS I Char/e5fifA/[md Paul 14 #0030 BY W -M ATTORNEY Aug. 8, 1961 P. A. HUDSON ET ALDRY STATIC CALORIMETER FOR RF POWER MEASUREMENT Filed Nov. 9, 1959 2Sheets-Sheet 2 215 D 200 /50 THERMOPILE 5 RESPONSE,

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ATTORNEY United States Patent 9 2,995,708 DRY STATIC 'CALORIMETER FOR RFPOWER Paul A. Hudson and Charles M. Allred, Boulder, (1010.,

assignors to the United States of America as represented by theSecretary of Commerce Filed Nov. 9, 1959, Ser. No. 851,925

3'Claims. (Cl. 324-106) This invention relates to wattmeters and moreparticularly to an improved wattmeter of the static calorimetric typefor measuring power primarily in the radio frequency range.

The measurement of RF power in standards work at frequencies below about500 me. with accuracies of 1 or better has been chiefly limited in thepast to the dynamic range which can be measured with bolometer orthermistor bridges. At the present time such bridges have an upper limitof approximately 100 milliwatts.

As is well known in the art, it is the aim of all calorimetric methodstodissipate completely the incoming electromagnetic energy in somemedium, using the effect on the medium as a measure of the incomingpower.

In direct-heating static calorimeters the calorimetric medium is itselfused to dissipate the electromagnetic energy. A prior-art, dry-loadcalorimeter for use in the microwave region consists of a coaxial linefilled with a high-loss dielectric; power is measured by the rate oftemperature rise in the dielectric. This type of calorimeter can also beextended to the lower frequency ranges by the use of materials that havesufiiciently high loss in the desired frequency band and which arecapable of withstanding the temperature rise.

In the indirect-heating static calorimeters a resistive load is immersedin a calorimetric fluid such as oil, water, or air. At low frequenciesload reactance can usually be made as low as desired and no problem ispresented if a resistive termination is employed. At radio frequencies,however, nonreactive loads are more diflicult to obtain and matchingsystems are frequently required to adjust the load to the desiredimpedance.

The wattmeter of this invention is a transfer standard betweenaccurately known values of DC. power and the RF power to be measured.Specifically, it is a highly accurate, simply instrumented,indirectly-heated static calorimeter having a dynamic range whichextends from 20 milliwatts to 12 watts and having a frequency range fromDC. to 300 me. Analysis of errors indicates a maximum uncertainty of*-(0.5% +0.2 mw.) in the measured RF power.

Because intercomparison of independent methods of measurement is highlydesirable in standards work, the range of the improved wattmeter of thisinvention permits its comparison with both the thermistor bridge and aliquid flow-type power meter. In comparison measurements with otherindependent methods, agreements of i0.5% or better were obtained. Thisaccuracy represents an improvement of one order of magnitude over thebest presently available commercial instruments designed for the abovepower and frequency range.

It is an object of this invention to provide an improved device for thestatic calorimetric measurement of power.

Another object of this invention is to provide an improvedstatic-calorimetric wattmeter which will measure power over a broad bandof frequencies with high efliciency.

A further object of this invention is to provide a calorimetricwattmeter which is capable of accurately rneas-.

uring power at all frequencies from direct current'through the radiofrequency range and which may be conveniently calibrated with directcurrent.

Other uses and advantages of the invention will become apparent uponreference to the specification and drawings in which:

FIG. 1 shows a vertical sectional view of a preferred embodiment of thestatic calorimeter of this invention;

FIG. 2 illustrates an enlarged vertical sectional view of the lowerportion of the calorimeter of FIG. 1;

FIG. 3 is a graph illustrating the thermopile output plotted as afunction of DC. power input to the calorimeter;

FIG. 4 is a graph illustrating thermopile sensitivity 'vs. power inputlevel; and

FIG. 5 is a graph of the voltage-standing-wave ratio (VSWR) of thecalorimeter as a function of frequency.

Referring now to the drawings, wherein like reference charactersdesignate like or corresponding parts throughout the several views,there is schematically shown in dotted line portions in FIG. 1 of thedrawings a thermally insulated chamber 10* providing a constanttemperature environment into which the dry, static calorimeter 11 ofthis invention is to be inserted.

The calorimetric power dissipating and measuring device illustrated inFIG. 1 is a designed for use in connection with coaxial lines. Forsupplying radio-frequency energy to the calorimeter 1 1 a conventionalline connector 12 is provided. An outer hollow cylindrical conductor 13having a constant impedance taper 13a is integral with the connector 12,said taper increases the line diameter to approximately 1 /2 inches andextends to slightly above the top surface of a cylindrical casing 14.

In a preferred embodiment of the invention the casing The top, bottomand side members 14a-14c, respectively, are one:

14 is constructed of aluminum or the like.

half inch in thickness and are attached by screws 15 or the like to forma cylindrical casing 4 inches in diameter A central tapered conductingrod 16, preferably con-i structed of brass or the like, is plated withsilver on its outer surface 16b. The rod 16 extends slightly below thelower surface of member 14a and terminates in the same horizontal planewith the conductor 13. A low-loss insulated bushing 17 supports theinner rod 16 in a conventional manner.

As illustrated in FIG. 1 of the drawings glass tubes 1819 having outsidediameters equal to those of con- 1 ductor 13 and rod 16, respectively,and approximately 4 inches in length, are soldered at one end to therespective elements 13 and 16 to form axial extensions thereto. Theinner surface 18a of the tube 18 and the outer sur-' face 1% of the tube19 are coated with a silver paint; such as Du Pont No. 4760, to athickness of approxi mately 0.001". The section of silvered glass lineprovides a fair degree of thermal isolation for the load re sistorhereinafter tobe discussed in detail.

Referring now to FIG. 2 of the drawings, the lower ends of the glasstubes 18-*19 are soldered in a con ventional manner to truncated annularrings 20-21 ofsilver, which in turn attach to a glass disc substrate 22.

A SO-o-hrn discatype resistor 23 which constitutes the nonreflectiveload or heat source is made by evaporating a thin film of a lowtemperature coefficient alloy onto the disc substrate 22 between therings 202 1. Contact to-the film is made by 'means of fired silverelectrodes. The thickness of the film is of the order of 10 cm. and

its temperature coefiicien't of resistance is approximately 20 parts permillion. per degree centigrade.

frequencies up to approximately 500 rnc., skin effect is Hence, at

negligible and at temperatures up to 100 C. the resistance changes byonly 0.2%. The change in reflected power due to such a small change inresistance is negligible in comparison to other uncertainties in thesystem.

A measure of the temperature rise in the load resistor 23 produced bythe dissipation of power in the load is obtained by means of aconventional copper-constantan thermopile 24 (see FIG. 1) consisting ofa plurality of series-connected thermocouples. In a preferred embodimentof the invention, the number of junctions was limited to 50, the neteflect of adding more thermocouples is to increase the sensitivity andalso the thermal conductance. The hot junctions 24a are soldered tosmall electrically isolated areas on the bottom surface of substrate 22whereas the cold junctions 241) are cemented to a ceramic disc 25 oflarge area. The hot and cold junctions 24a-2 4b are positionedapproximately one inch apart. Disc 25 is preferably constructed of A1and is cemented to the bottom member 14b of the easing 14 as bySauereisen Cement No. 29 or the like.

The thermopile leads 24c connect with and are brought through the casingby 1000 mmfd. feed-through capacitors 26, such as Aerovox type CF-l,which screwably insert into the lower side portions 140 of the casing.

Casing 14 is supported within the container by a plurality of legs 27which attach to the bottom member 14b in a conventional manner.

In the static calorimeter where high accuracy is desired, thetemperature rise is usually measured at conditions of thermal steadystate. Since a measurement of temperature rise only is desired, thesteady state temperature T may be measured with respect to somearbitrarily chosen reference temperature T In a preferred embodiment ofthe invention, chamber 10 contains a refrigeration coil (not shown) overwhich air is continually recirculated. The temperature within chamber 10is 0 C. and is kept constant to within *-0.002 C. by means of asensitive electronic circuit. The calorimeter 11 is thus kept at thetemperature of the chamber 10, said temperature serves as the referencetemperature T In the absence of power input the equilibrium temperatureof the entire calorimeter will, of course, be T also.

The steady-state temperature T is a function of power input as well asthe degree of thermal isolation of the body from its surroundings.Temperature T is independent of frequency therefore the calorimeter canconveniently be calibrated with D.C. power and subsequently used tomeasure power at any frequency. Thus the calorimeter is actually atransfer standard measuring RF power in terms of accurately known D.C.or low frequency power.

When power is fed into the input of the calorimeter at a fixed level,heat is generated in the load resistor at the same rate as theabsorption of the electrical energy. The heat is first manifested in atemperature rise of the load resistor which has a certain thermalcapacity. Subsequently, heat flows from the load to the surroundings byconduction, convection and radiation. After equilibrium has beenattained, the thermal is measured across the terminals 26a-26b ofcapacitors 26 with a precision D.C. potentiometer. The thermal is thencalibrated in terms of the applied power.

The measured response of the thermopile, in millivolts, is plotted inFIG. 3 of the drawings as a function of the D.C. power input in watts.Experimental data shows that below 0.2 watt the curve is approximatelylinear while between 0.2 watt and 1 watt the curve is defined by thesimple empirical equation where e represents the open circuit thermopileoutput voltage, K is a constant of proportionality and P is the D.C.power input in watts. Equation 1 plots as a straight line on log logpaper.

Above 1 Watt the equation of the curve of FIG. 3 is K p0.92'1 (2) whichproduces another straight line as a log log plot.

The sensitivity of the improved calorimeter of this invention is 23.50mv. per volt below 0.2 watt and as shown by the plots in FIG. 4decreases at higher power levels due to convection and radiation losses.The equation of the curve in FIG. 4 of the drawings is where S is thesensitivity of the calorimeter at any power level, S is the initialsensitivity and P is power in watts. At 12 watts, for example, the valueof S is approximately 17% below S Since the calorimeter is housed in atemperature controlled environment ordinary room temperature variationshave no noticeable efiect on the thermopile output. There is, however, asmall residual output from the thermopile of i5 microvolts with zeropower input to the calorimeter. This residual is equivalent to 10.2 mw.and causes this much uncertainty in the measurement at all power levels.

The time constant of the calorimeter, which is defined as the timenecessary for the thermal to reach 63% of its final value, isapproximately 4 minutes. To obtain maximum accuracy, however, a periodof approximately 40 minutes must be allowed between measurements topermit the system to reach steady state conditions.

As aforementioned, this calorimetric wattmeter is not an absoluteinstrument but rather the response of the temperature-sensing devicemust be calibrated using known values of D.C. or low frequency power.For example, the calorimeter may be calibrated with D.C. power using alaboratory standard type voltmeter and ammeter. The accuracy of each ofthese instruments is 0.1%, hence the accuracy of the D.C. power was0.2%.

At frequencies up to 300 me. losses in the coaxial mount were estimatedto be 0.0025 db or 0.05 This estimate is based on the values given intables for 1 /2 inch diameter rigid coaxial line with air dielectric.Thus, in using the calorimeter to measure RF power the uncertainty inthe measurements amounts to i (0.5%+0.2 mw.)

this factor includes the substitution error, the D.C. calibration erroras well as losses in the coaxial mount but.

does not include errors due to the VSWR being different from unity.

The calorimeter measures, of course, only the power it absorbs. Someenergy is reflected at RF since the VSWR is not unity. Since the VSWR isknown, a correction factor due to reflected power can be applied; asillusrated in the graph of FIG. 6, the VSWR varies with frequency.

Performance of the calorimeter was further evaluated by makingcomparison RF power measurements between it and other independentmethods. These included a low-.

power thermistor bridge and a medium power bolometer bridge. In allcases agreement was equal to or better than the sum of the uncertaintiesof the instruments involved.

It should be understood, of course, that the foregoing disclosurerelates to only a preferred embodiment of the invention and that it isintended to cover all changes and modifications of the example of theinvention herein chosen for the purposes of the disclosure, whichv donot constitute departures from the spirit and scope of'the invention.

What is claimed is:

1. In a static calorimetric wattmeter, a coaxial transmission lineincluding a central conducting rod, a coaxial outer conductor and aninput connector, said outer conductor having a constant impedance taper,-a casing consisting of top, bottom and side members, said outerconductor connecting with said top casing member, a disc substratehaving first and second surfaces, means connecting the outer conductorand the central conducting rod to the first surface of said discsubstrate, a load resistance film on said first surface of the substrateto thereby bridge the coaxial conductors and terminate said coaxialline, a plurality of thermocouples having hot and cold junctions, aceramic disc on said bottom casing member, the hot junctions of saidthermocouples attached to said second surface of the disc substrate andthe cold junctions attached to said ceramic disc.

2. In a static calorimetric wattmeter, a coaxial line including acentral conducting rod, a coaxial outer conductor and an inputconnector, said outer conductor having a constant impedance taper, athick-walled casing consisting of top, bottom and side members, meansconnecting said outer conductor with said top casing member, a discsubstrate having first and second surfaces, means connecting said outerconductor and said central conducting rod to said first surface of thedisc substrate, a resistive film comprising a non-reflective load onsaid first surface of the disc substrate to thereby bridge the coaxialconductors and terminate the coaxial line, a plurality of seriesconnected thermocouples having hot and cold junctions, said hotjunctions of said thermocouples attached to said second surface of thedisc substrate, a ceramic disc positioned on said bottom casing member,said cold junctions of said thermocouples attached to said ceramic disc.

3. In a static calorimetric wattmeter, a coaxial transmission lineincluding a central conducting rod, a coaxial outer conductor and aninput connector, said outer conductor having a constant impedance taper,a thick-walled casing consisting of top, bottom and side members, saidouter conductor connecting with said top casing member, a disc substratehaving first and second surfaces, means axially extending said outerconductor and central conducting rod, said axial extensions attached tosaid first disc substrate surface, a resistive film comprising anonreflective load on said first substrate surface to thereby bridge theconductors and terminate the coaxial line, a plurality of seriesconnected thermocouples having hot and cold junctions, a ceramic discattached to said bottom casing member, said hot junctions of saidthermocouples attached to said second disc substrate surface and saidcold junctions attached to said ceramic disc.

References Cited in the file of this patent UNITED STATES PATENTS2,848,683 Jones Aug. 19, 1958

